1. Field of the Invention.
This invention is in the field of solar collectors.
2. Description of the Prior Art.
Whitaker (App. No. 06/521,491 Filed 10/09/79) discusses a solar collector using a matrix of lenses and a set of optical fibers, each lens focusing solar radiation upon a respective optical fiber. The entire array of lenses is oriented to face the sum. Consequently each solar image is kept on the optical axis of its respective lens at all times.
Diner (U.S. Pat. No. 4,201,197) discusses a similar system, using one large lens focusing upon the open end of a bundle of optical fibers.
Bowers (U.S. Pat. No. 4,282,858 filed 3/27/80) discusses a simlar system to that of Whitaker.
The French and Japanese are both working significantly in this field. No publications are presently on hand.
Pertinent details regarding flat plate collectors and the Whitaker system will next be discussed.
A typical installation of conventional flat plate solar collectors is shown in FIG. 1. The collectors 11, 12, and 13 are placed on a south-facing roof. Solar radiation passes through the glass plate with which each collector is covered. Within the collector this radiation is absorbed on a blacken surface and turned into sensible heat. A portion of this heat passes to a working fluid circulated through the collector and is by that fluid carried to a using device (such as a hot water heater) inside the house. A second portion is lost by conduction back up through the glass to the ambient air. As the ambient temperature drops, this loss becomes excessive. A typical collector will lose half the collected energy via this conduction when the outside temperature drops to 273 K (freezing). Consequently conventional flat plate collectors are useful for heating water and homes only in temperate or tropic climates.
A second significant feature of the flat plate collector is that the maximum temperature it can achieve in the working fluid is about 350 K. This is achieved with a gray body absorber and a solar constant of 800 w/m.sup.2. A spectrally selective surface permits a higher temperature to be attained. But such surfaces are expensive. As a consequence they are not much used. At 350 K the surface reradiates as much power in infrared radiation as it absorbs in solar radiation. If there is to be significant power delivered to a using device then the absorber must operate at a temperature below 350 K.
Whitaker Fiber Optic Collector. FIG. 2 shows a conventional fiber optic collector such as that disclosed by Whitaker, application Ser. No. 06/521,491. A mosaic of lenses 21 faces the sun. Solar radiation 22 passes through lenses 21 and forms solar images on the open ends of optical fibers 23. Optical fibers 23 are bundled together and led to a using system such as a hot water heater.
A significant feature of this system is that there is no conduction loss to the surrounding air. The heat which this system delivers is a function of the solar radiation impinging on the collector and that only. Consequently it can deliver as much heat on the coldest day of winter as it does on the hottest day of summer. This is in stark contrast to the conventional flat plate collector--which can deliver no heat on the coldest day of winter when that heat is needed most.
A second significant feature is that heat can be delivered at high temperature. It can be shown that the theoretical limit is the temperature of the surface of the sun--some 6,000 K. This can be achieved by operating the system outside the atmosphere, using lossless materials, and using an absorber such as that described in Whitaker U.S. Pat. No. 3,234,931.
A third significant feature is that an orientation system is required. It must turn the entire array to face the sun. Orientation systems are available but they are trouble prone and costly. This is especially true in location where storms are common.